First accelerate particles to near the speed of light, and then let the particles enter a decelerating electric field.
If the particles decelerate very slowly(very slowly=in accordance with special relativity), it proves that the speed/mass relationship of the theory of relativity is correct.
If the particles decelerate very quickly(very quickly= more quickly than predicated by special relativity), it proves that the speed/mass relationship of the theory of relativity is wrong.
Because, according to theory of relativity, the mass/energy/momentum(inertia) of particles will increase dramtically at near the speed of light, the particles will be decelerated very slowly inside decelerating electric field due to their hugely increased mass/energy/momentum(inertia). Thus if the particles can be decelerated very quickly, this means the mass/energy/momentum(inertia) of the particles has not increased. Therefore the speed/mass relationship of the theory of relativity must be incorrect.
Detailed discussion can be found at: 10.5281/zenodo.4781247
Thank you very much for your kind attention.(If you can actually do this experiment with or without my participation, I will be very happy to see it happen)